1. Field of the Invention
The present invention relates generally to speedometers for vehicles and, more particularly, to a method of selecting a pinion for a speedometer on an automotive vehicle.
2. Description of the Related Art
Currently, on front wheel drive automotive vehicles, vehicle speed information for a vehicle speedometer system is provided by a distance sensor (speed sensor) mounted in the extension housing of a transmission. The sensor is driven directly by a worm gear machined into the right driveshaft of the transmission and generates a predetermined number of pulses such as eight pulses per sensor shaft rotation. The desired distance signal of 8000 pulses per mile is achieved through the use of a plastic pinion gear on the sensor shaft. Pinion gears of varying tooth count are installed in assembly plants to compensate for different tire sizes and are selected based on the sales codes on the vehicle's build sheet.
The signal generated by the distance sensor is supplied to an engine controller and a mechanical instrument cluster (non serial data or CCD bus). The engine controller provides a count of speed pulses as a message on the CCD bus every 344 milliseconds. This message is utilized by body controllers, electronic instrument clusters, and other electronic modules in indicating or calculations using vehicle speed or distance traveled.
One disadvantage of the above is that a distance sensor and pinion gear are needed to provide vehicle speed information, resulting in increased cost to the vehicle. Another disadvantage is that several different sized pinion gears are needed to compensate for various tire sizes and the improper size pinion gear may be accidentally installed in the assembly plant.